Subterranean formation heating using electromagnetic energy relates to the technology for heating of bitumen and/or heavy oil in oil-sand mediums using radio frequency (electromagnetic) energy. Radio frequency heating uses antennas or electrodes to heat the buried formation. This enables a quick and efficient heating of hydrocarbons by coupling antennas into the formation. As a result, the heated hydrocarbons become less viscous which aids in oil production.
Materials such as oil shale, tar sands, and coal are amenable to heat processing to produce hydrocarbon liquids. Generally, the heat develops the porosity, permeability, and/or mobility necessary for recovery. Oil shale is a sedimentary rock, which upon pyrolysis, or distillation, yields a condensable liquid, referred to as a shale oil, and non-condensable gaseous hydrocarbons. The condensable liquid may be refined into products that resemble petroleum products. Oil sand is an erratic mixture of sand, water, and bitumen, with the bitumen typically being present as a film around water-enveloped sand particles. Though difficult, various types of heat processing can release the bitumen, which is an asphalt-like crude oil that is highly viscous.
A number of proposals, broadly classed as in-situ methods, have been made for processing and recovering hydrocarbon deposits. Such methods may involve underground heating of material in place, with little or no mining or disposal of solid material in the formation. Useful constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injection techniques. For such methods to be successful, the amount of energy required to effect the extraction should be minimized.
One proposed electrical in situ approach employs a set of arrays of dipole antennas located in a plastic or other dielectric casing in a formation, such as a tar sand formation. A VHF or UHF power source would energize the antennas and cause radiating far fields to be emitted into the deposit. However, at these frequencies, and considering the electrical properties of the formations, the field intensity drops rapidly as distance from the antennas increases. Consequently, non-uniform heating results in inefficient overheating of portions of formations to obtain at least minimum average heating of the bulk of the formation.
Many efforts have been attempted or proposed to heat large volumes of subsurface formations in situ using electric resistance, gas burner heating, steam injection and electromagnetic energy, such as to obtain kerogen oil and gas from oil shale. Resistance type electrical elements have been positioned down a borehole via a power cable to heat the shale via thermal conduction. Unfortunately, the thermal conductivity of oil sand is low, under about 2 watts/meter degree Kelvin so conducted heat flow is slow. Electromagnetic energy has been delivered via an antenna or microwave applicator. The antenna is positioned down a borehole via a coaxial cable or waveguide connecting it to a high-frequency power source on the surface. Subterranean formation heating is accomplished by eddy currents, radiation and dielectric absorption of the energy of the electromagnetic (EM) wave radiated by the antenna or applicator. This may be better than more common resistance heating which relies solely on conduction to transfer the heat. It is also better than steam heating which requires large amounts of water and energy present at the site.
U.S. Pat. No. 4,140,179 discloses a system and method for producing subsurface heating of a formation comprising a plurality of groups of spaced RF energy radiators (dipole antennas) extending down boreholes to oil shale. The antenna elements should be matched to the electrical conditions of the surrounding formations. However, as the formation is heated, the electrical conditions can change whereby the dipole antenna elements may have to be removed and changed due to changes in temperature and content of organic material.
U.S. Pat. No. 4,508,168 describes an RF applicator positioned down a borehole supplied with electromagnetic energy through a coaxial transmission line whose outer conductor terminates in a choking structure comprising an enlarged coaxial stub extending back along the outer conductor.
However, RF currents flow along the outside of the coaxial cable (e.g. common mode current) and result in unwanted overburden heating or even undesired surface heating. The conventional sleeve baluns or common mode chokes are intended to stop the unwanted current but existing balun chokes are too long and may preclude or impede surface operation. Bending the choke at the surface reduces the effectiveness as stray capacitance to the antenna allows RF currents to circumvent the balun. Also, a bent balun may still present an oversize structure requiring excessive wellpad area. Thus, a shorter balun choke is desired.